1. Field of the Invention
The present invention relates to a brushless DC fan motor suitable for a fan radiating heat from a housing of an OA appliance, and more particularly to a current limiting circuit thereof.
2. Description of the Related Art
In an electronic appliance such as OA appliances including personal computers and copiers, a large number of electronic components are accommodated in a small housing, whereby heat is generated from the electronic components and accumulated in the housing so as to have a possibility to damage the electronic components.
To solve this problem, a ventilation hole is provided in a wall or a ceiling of the housing, and a fan motor is mounted in the ventilation hole so as to radiate heat outside of the housing.
In the fan motor of the above, a noise occurs when the fan motor is started (hereinafter referred to as a xe2x80x9cstarting noisexe2x80x9d), and the starting noise will be maximum when relatively large fan motor is provided, causing the harsh grating noise.
When the brushless DC fan motor is used for the fan motor, the staring noise becomes prominent especially when the ON-OFF frequency of the starting current is less than 1 kHz.
The brushless DC fan motor is not exceptional among other motors so that a current limiting circuit is provided as a countermeasure in order to prevent the excess current from being occurred when the fan motor is started or in an overload condition.
Although the current limiting circuit is employed in such a manner as to shut off field coils to be electrified to suppress the excess current, such a ON-OFF frequency of the starting current largely depend on the configuration of the current limiting circuit.
FIG. 3 shows the current limiting circuit of a conventional brushless DC fan motor.
In the figure, numeral 31 denotes current limiting circuit of a brushless DC fan motor (circuit) 32. Here shows the current limiting circuit 31 in the two-phase motor 32.
As shown in the figure, the brushless DC fan motor 32 comprises field coils L1 and L2, FETs T1 and T2, and a drive circuit DRV. The field coils L1 and L2 are mounted on a stator (not shown), and conducted by the FETs T1 and T2 in an alternately switching manner to form the rotating magnetic field. A rotor (not shown) rotates by means of the rotation of a permanent magnet provided thereon following the above rotating magnetic field.
The current limiting circuit 31 comprises resistors R0 to R3, capacitors C1 and C2, a comparator COM, a PNP transistor Q1, an NPN transistor Q2 and diodes D1 and D2.
The resistor R0, which is a current-detecting resistor connected in series to the field coils L1 and L2 so as to detect the current in the field coils L1 and L2, detects the current by converting into a voltage VA produced across the resistor.
The comparator COM compares a control voltage VB from the current-detecting resistor R0 side with a reference voltage Vf and outputs a L (Low) level signal when the control voltage VB exceeds the reference voltage Vf. The reference voltage Vf is the voltage corresponding to an allowable maximum value If of the preset starting current.
When an output signal VCxe2x80x2 of the comparator COM is on the L level, the transistors Q1 and Q2 are turned on, and the diodes D1 and D2 are forward-biased to realize the conduction.
As a result, each gate G of the FETs T1 and T2 is grounded via the diodes D1 and D2 and the transistor Q2, and thus, the FETs T1 and T2 are turned off irrespective of the state of the control signal from the drive circuit DRV and shut off the field coils L1 and L2 to be electrified. This means that the current is limited when the current in the field coils L1 and L2 exceeds the allowable maximum value If of the starting current.
The OFF state of the FETs T1 and T2 is continued so long as the control voltage VB exceeds the reference voltage Vf and limits the starting current. When the control voltage VB is less than the reference voltage Vf, the output signal VCxe2x80x2 of the comparator COM is on the H (High) level, the transistors Q1 and Q2 are turned off, and the diodes D1 and D2 are reverse-biased to realize the non-conduction. As a result, each gate G of the FETs T1 and T2 is released from the grounded state, and the FETs T1 and T2 are controlled by the signal from the drive circuit DRV. This means that the rotation is returned to normal one.
However, in the above conventional technology, the ON-OFF frequency of the starting current is less than 1 kHz when the motor is started, and a problem regarding the harsh grating noise of the starting noise remains.
This problem will be described below. In FIG. 4, a waveform (I) shows a normal waveform of the electrified current of the field coils L1 and L2 in the brushless DC fan motor 32. However, this waveform (I) is the synthesized current waveform of the field coils L1 and L2, in other words, the waveform of the electrified current in the current-detecting resistor R0. The ON-OFF frequency of the starting current shown in the waveform (I) in FIG. 4 is not less than 1 kHz.
In such a waveform (I) of the electrified current, the waveform of the output signal VC of the comparator COM (the voltage waveform of the anode of the diodes D1 and D2) is shown in (VC) in FIG. 4 if the current limiting circuit 31 works normally where the level If is the allowable maximum level of the predetermined current, i.e., the starting current. This means that the frequency of the output signal VC of the comparator COM (the inverting frequency on the L and H levels) is maintained to be at least 1 kHz, and the starting noise does not form any harsh grating noise.
However, in the conventional technology, the reference voltage Vf is set to be a small value. This is because the current-detecting resistor R0 cannot be increased over a predetermined value, and as a result, the voltage VA detectable by the resistor R0 cannot be increased either for the reason that the magnitude of the electrified current to the field coils L1 and L2 is not limited.
As a result, both the control voltage VB and the reference voltage Vf for the comparator COM are decreased, and the comparison of high accuracy cannot be realized in the comparator COM. Thus, the comparator is easily affected by various kinds of operation disturbance factors such as the change in temperature, the change in source voltage and the noises from the inside and outside of the current limiting circuit 31, and the waveform of the output signal VC of the comparator COM actually becomes what is shown in (VCxe2x80x2) in FIG. 4.
This means that the H-level portion in the waveform shown in (VC) in FIG. 4 becomes a randomly-chipped waveform, which means that the ON-OFF frequency of the starting current is decreased, i.e., less than 1 kHz, causing the starting noise.
Similar phenomena occur when the motor is forcibly stopped by the external force or the like, and the improvement of these problems has been requested.
Accordingly, the present invention has been made in light of the above problems, and an object of the present invention is to provide a current limit circuit of a brushless DC fan motor to eliminate the harsh grating noise attributable to the reduction of the current ON-OFF frequency from a normal range, for example, below 1 kHz when the motor is started, or forcibly stopped by the external force.
In order to solve the above problem, there is provided a current limiting circuit in a first aspect of the present invention which limits the current in field coils of a brushless DC fan motor comprising a stator having a plurality of the field coils and a rotor having a permanent magnet in which the rotor is rotated by alternately switching the current in the field coils and rotating the permanent magnet following the rotating magnetic field formed by the field coils by shutting off the field coils to be electrified for a period in which the control voltage exceeds the reference voltage by the output signal of a comparator which compares the control voltage as the voltage according to the voltage produced across a current-detecting resistor interposed in electrified passages of the field coils with the preset reference voltage, and comprises a differential amplifier which is interposed between the current-detecting resistor and the comparator, amplifies the voltage produced across the current-detecting resistor and applies the voltage to the comparator as the control voltage.
There is provided a current limiting circuit in a second aspect of the present invention which limits the current in field coils of a brushless DC fan motor comprising a stator having a plurality of the field coils with each switching element connected in series thereto and a rotor having a permanent magnet in which the rotor is rotated by alternately switching the current in the field coils by turning ON/turning OFF the switching elements and rotating the permanent magnet following the rotating magnetic field formed by the field coils by shutting off the field coils to be electrified for a period in which the control voltage exceeds the reference voltage by the output signal of a comparator which compares the control voltage as the voltage according to the voltage produced across a current-detecting resistor interposed in electrified passages of the field coils with the preset reference voltage, and comprises a differential amplifier which is interposed between the current-detecting resistor and the comparator, amplifies the voltage produced across the current-detecting resistor and applies the voltage to the comparator as the control voltage, and switch elements which arc provided corresponding to each of the switching elements, receive the output signal of the comparator and shut off the field coils to be electrified with a control input terminal of the switching elements connected in series to the field coils on the supply side of the current to produce the control voltage exceeding the reference voltage for a period in which the control voltage exceeds the reference voltage.